Process for producing alkenyl aromatic foams using a combination of atmospheric and organic gases

ABSTRACT

A process for producing alkenyl aromatic foams utilizing a combination of atmospheric and organic gases as blowing agent, preferably using greater than 30% by weight of atmospheric gas, and preferably also using a predetermined about of a masterbatch mix comprising a styrenic polymer, a rubbery block copolymer, and a solid blowing agent. Also disclosed are alkenyl aromatic foams produced by the process which exhibit increased densities, increased thermoforming capabilities, increased post-expansion properties, and increased retainment of the atmospheric and organic gases.

This is a continuation, of application Ser. No. 07/995,289, filed Dec.22, 1992, now U.S. Pat. No. 5,269,987.

FIELD OF THE INVENTION

This invention relates to a process for producing alkenyl aromatic foamsutilizing a combination of atmospheric and organic gases as blowingagents, preferably also using a predetermined amount of masterbatch mix.The invention also relates to alkenyl aromatic foams resulting from theprocess, and articles made therefrom. Preferably, the alkenyl aromaticis polystyrene. The process and resulting foams of the present inventionare conferred with several benefits among which are an increase in theproduction rate of the process, a reduction in the amount of organic gaswhich must be used in the process in order to obtain a foamed producthaving a given density, an increase in the thermoforming output of thefoams due to, inter alia, an increase in the post-expansion propertiesof the foams, and an increase in retainment of the atmospheric andorganic gases in the foamed product.

BACKGROUND OF THE INVENTION

A variety of normally gaseous or liquid blowing agents have beenproposed for olefinic or styrenic polymers, including virtually all ofthe common atmospheric gases and lower hydrocarbons.

Alkenyl aromatic foams, particularly polystyrene foams in sheet form,are presently being made from a number of blowing agents which have manyundesirable characteristics. Volatility, flammability, poorthermoforming qualities, brittle physical properties, high cost, or anadverse affect to the ozone layer are just a few. Examples of theblowing agents that produce these characteristics in the production ofpolystyrene foam would include the aliphatic hydrocarbons, and fully (orpartially) halogenated hydrocarbons.

For polystyrene, for example, the C₄ -C₆ alkanes have gatheredwidespread acceptance, especially pentane. Following a typical extrusionfoaming step, the stock material is ordinarily aged before thermoforminginto containers or the like. During aging, the foam cells and polymericmatrix become partially depleted of volatile hydrocarbons, which enterthe atmosphere. However, potential atmospheric contamination by theseby-products of foam manufacture has led workers to seek non-pollutingalternative blowing agents, such as the usual atmospheric gases, e.g.,nitrogen and carbon dioxide, and combinations of atmospheric gases withorganic gases, e.g., the lower hydrocarbons or the freons.

In the prior art, both atmospheric gases, per se, and combinations ofatmospheric and organic gases have been disclosed as blowing agents foralkenyl aromatic polymers.

Australian Patent Application No. 52724/79, published Canadian PatentApplication No. 2,022,501 and published European Patent Application No.0,411,923 all disclose blowing agents consisting of carbon dioxide foralkenyl aromatic or styrenic polymers. The resulting foamed products aresaid to be flexible and/or have improved tensile elongation properties.However, the production rates of the processes are generally low, on theorder of less than 200 lbs./hr., and also have generally lowpost-expansion properties, on the order of 50% or less. In addition,these processes require relatively high extrusion temperatures, on theorder of 130° C. to 155° C. Thus, these processes are not veryeconomical.

In co-pending patent application Ser. No. 07/891,866, there aredisclosed processes for producing polystyrene foams utilizing 100% ofatmospheric gas, e.g. carbon dioxide and/or nitrogen, which can beeffected at a much lower extrusion temperature, i.e. on the order ofabout 120° C., utilizing in the melted polymer an additive comprised ofa masterbatch mix containing alpha-methyl polystyrene, a rubbery blockcopolymer, a solid blowing agent comprised of an encapsulatedcombination of monosodium citrate and sodium bicarbonate, white mineraloil, and silica.

U.S. Pat. Nos. 4,344,710 and 4,424,287 disclose blowing agents which arecombinations of liquid carbon dioxide and liquid aliphatic, or fully (orpartially) halogenated hydrocarbons. These patents state that the use ofatmospheric gases, including 100% carbon dioxide or nitrogen as blowingagents has not been successfully employed, giving as a reason theextreme volatility of these gases, and further state that the use ofthese materials is said to produce corrugation and surface defects inthe sheet product. These two patents disclose that a combination ofatmospheric and organic gases, in an alkane: CO₂ feed ratio in the rangeof 3:1 to 1:1 by weight, can be used, with the total amount of blowingagent combination being in the range of 2.5 to 10 parts per 100 parts byweight of thermoplastic resin. As nucleating agents for the foamedproducts, the patents disclose the use of a mixture of sodiumbicarbonate and citric acid. The process temperatures needed forextrusion of the foam are again quite high, on the order of 150° C.

U.S. Pat. No. 4,424,287 further discloses that the foams prepared withthe combination of blowing agents exhibit the advantage of reducedatmospheric emissions upon aging without, however, any data to thiseffect, merely stating that the reduction in pollutant (i.e. thehydrocarbon blowing agents) is greater than the expected reduction dueto the corresponding decrease in organic blowing agent use. The onlyrationale provided in U.S. Pat. No. 4,424,287 for the reducedhydrocarbon emissions is the ability of the foamed sheet product to beimmediately thermoformed, thereby reducing the need for aging of thefoamed sheet product.

U.S. Pat. No. 4,419,309 discloses the use of two foaming agents; thefirst being introduced into a molten thermoplastic resin under higherpressure, with the first foaming agent being selected from a lowmolecular weight aliphatic hydrocarbon, a low molecular weighthalocarbon and mixtures thereof, and the second foaming agent beingintroduced under lower pressure, with the second foaming agent beingselected from carbon dioxide, water vapor and mixtures thereof, to causefoaming of the melted thermoplastic resin. Again, the extrusion ratesare low, on the order of 150 lbs./hr., and the extrusion temperaturesare high, on the order of 290° F.-320° F.

U.S. Pat. Nos. 4,916,166 and 5,011,866 disclose alkenyl aromaticthermoplastic synthetic resinous elongated foam bodies having a machinedirection, a transverse direction and closed, non-interconnectinggas-containing cells, which are prepared using, preferably at least 70%by weight of 1,1-difluoro-1-chloroethane (U.S. Pat. No. 4,916,166) andrequiring the use of at least 70% by weight of 1,1,1,2-tetrafluoroethaneor 1,1,1-trifluoroethane, based on the total weight of blowing agentmixture weight (U.S. Pat. No. 5,011,866), and using as a second blowingagent up to 30 weight percent (of the blowing agent in an amount ofmixture) chemical or physical blowing agents, including water, 1-4carbon aliphatic hydrocarbons, carbon dioxide, or otherhydrogen-containing chlorofluorocarbons (HCFCs) such aschlorodifluoromethane (HCFC-22).

U.S. Pat. No. 4,916,166 discloses that the amount of carbon dioxide islimited to no more than about 6% by weight and that extruded articleshaving densities between 2.4 and 5.0 pounds per cubic foot may beobtained only by extrusion at a die temperature of about 118° C. orless. The extrusion rate at this temperature should necessarily be quitelow, although the patent is silent on this point. The specific examplesof U.S. Pat. No. 4,916,166 show that extruded foam articles havingdensities of less than 2.4 pounds per cubic foot are obtained only uponextrusion above 118° C., and these are obtained utilizing blowing agentswhich contain only about 2.7% by weight carbon dioxide based upon 100%by weight of the blowing agent mixture.

U.S. Pat. No. 5,011,866 discloses alkenyl aromatic thermoplasticsynthetic resinous elongated foamed products having densities of fromabout 1 to about 6 pounds per cubic foot which have a plurality ofclosed non-interconnecting gas-containing cells, with the limitationthat the cells contain at least 70% by weight of either1,1,1-trifluoroethane or 1,1,1,2-tetrafluoroethane. U.S. Pat. No.5,011,866 likewise prefers the use of less than 6% carbon dioxide as acomponent in a blowing agent mixture although some examples show the useof about 9% carbon dioxide.

Thus, there still exists a need in the art for procedures for theproduction of alkenyl aromatic foams which utilize combinations ofatmospheric and organic gases as blowing agents and having an increasedamount of atmospheric gas, such as carbon dioxide or nitrogen. Therealso still exists a need in the art for such alkenyl aromatic foamswhich can be produced at increased temperatures and increased extrusionrates for a given density. Still further, there exists a need in the artfor alkenyl aromatic foams having an increased percentage of closed,non-interconnected cell structure, increased post-expansion properties,and increased retainment of blowing agent within the cell structure ofthe alkenyl aromatic foam.

These and other needs still remaining in the alkenyl aromatic foam artare met and satisfied by applicants' present invention, described below.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided a processfor the production of alkenyl aromatic foams, said process comprising:

(a) heating an alkenyl aromatic resin to a temperature above its meltingpoint to form a melted alkenyl aromatic resin;

(b) adding to the melted alkenyl aromatic resin a masterbatch mixcomprised of:

(i) encapsulated monosodium citrate and sodium bicarbonate;

(ii) styrene-ethylene/butylene-styrene block copolymer;

(iii) alpha-methyl polystyrene;

(iv) white mineral oil; and

(v) silica, to form an alkenyl aromatic/masterbatch mix blend;

(c) heating the alkenyl aromatic/masterbatch mix blend to a temperaturesufficient to form a melted blend;

(d) injecting into the melted blend a non-solid blowing agent comprisedof a combination of atmospheric gas and organic gas to form an injectedmelted blend;

(e) mixing the injected melted blend to form a mixed injected meltedblend; and

(f) cooling and extruding the mixed injected melted blend as an alkenylaromatic foam.

In another embodiment of the present invention, there is provided analkenyl aromatic foam composition comprised of:

(a) an alkenyl aromatic polymer;

(b) alpha-methyl polystyrene;

(c) styrene-ethylene/butylene-styrene block copolymer;

(d) white mineral oil;

(e) the decomposition products of an encapsulated monosodium citrate andsodium bicarbonate; and

(f) silica, wherein the foam is comprised of closed cells containingtherein a combination of atmospheric gas and organic gas.

In still a further embodiment of the present invention, there isprovided an extruded alkenyl aromatic foam having a density greater than2.5 pounds per cubic foot, having been extruded at a die temperature of120° C. or greater, and having a plurality of closed non-interconnectedgas-containing cells therein, wherein the gas contained in the cells iscomprised of a combination of atmospheric gas and organic gas andwherein the atmospheric gas is present in an amount of at least 30% byweight, based upon the total weight of the gas contained in the cells.

In a still further embodiment, the present invention provides for aprocess for the production of alkenyl aromatic foams having a density ofgreater than about 2.5 pounds per cubic foot, said process comprising:

(a) heating an alkenyl aromatic resin to a temperature above its meltingpoint to form a melted alkenyl aromatic resin;

(b) adding to the melted alkenyl aromatic resin a masterbatch mixcomprised of:

(i) a styrene resin;

(ii) a rubbery block copolymer; and

(iii)a solid blowing agent to form an alkenyl aromatic/masterbatch mixblend;

(c) heating the alkenyl aromatic/masterbatch mix blend to a temperaturesufficient to form a melted blend;

(d) injecting into the melted blend a non-solid blowing agent comprisedof a combination of atmospheric gas and organic gas to form an injectedmelted blend, wherein the atmospheric gas is present in an amount of atleast about 30% by weight based upon the total weight of atmospheric gasand organic gas;

(d) mixing the injected melted blend to form a mixed injected meltedblend;

(e) cooling the mixed injected melted blend; and

(f) extruding the cooled blend at a temperature not below 120° C. as analkenyl aromatic foam.

In yet a still further embodiment of the present invention, there isprovided an alkenyl aromatic foam having a density of greater than 6.0pounds per cubic foot, having a plurality of closed, non-interconnectinggas-containing cells therein, wherein the gas contained in the cells iscomprised of atmospheric gas and organic gas.

In yet another further embodiment of the present invention, there isprovided a process for the production of alkenyl aromatic foams having adensity of greater than 6.0 pounds per cubic foot, said processcomprising:

(a) heating an alkenyl aromatic resin to a temperature above its meltingpoint to form a melted alkenyl aromatic resin;

(b) adding to the melted alkenyl aromatic resin a masterbatch mixcomprised of:

(i) encapsulated monosodium citrate and sodium bicarbonate;

(ii) styrene-ethylene/butylene-styrene block copolymer;

(iii) alpha-methyl polystyrene;

(iv) white mineral oil; and

(v) silica, to form an alkenyl aromatic/masterbatch mix blend;

(c) heating the alkenyl aromatic/masterbatch mix blend to a temperaturesufficient to form a melted blend;

(d) injecting into the melted blend a non-solid blowing agent comprisedof a combination of atmospheric gas and organic gas to form an injectedmelted blend;

(e) mixing the injected melted blend to form a mixed injected meltedblend;

(f) cooling the mixed injected melted blend; and

(g) extruding the cooled blend as an alkenyl aromatic foam.

DETAILED DESCRIPTION OF THE INVENTION

The polyalkenyl aromatic polymers can be, for example, styrene polymers.The styrene polymers included in the compositions of the invention arehomopolymers of styrene and copolymers and interpolymers of styrenecontaining a predominant proportion of styrene, e.g. greater than 50weight percent, and preferably greater than 75 weight percent, styrene.Examples of monomers that may be interpolymerized with the styreneinclude alpha, beta-unsaturated monocarboxylic acids and derivativesthereof, e.g. acrylic acid, methyl acrylate, ethyl acrylate, butylacrylate, 2-ethylhexyl acrylate and the corresponding esters ofmethacrylic acid, acrylamide, methacrylamide, acrylonitrile,methacrylonitrile, maleic anhydride, etc. If desired, blends of thestyrene polymer with other polymers may be employed, e.g. blends of thestyrene polymer with grafted rubbery diene polymers, or the analogouscompositions obtained by dispersing a rubber diene polymer in thestyrene monomer and, optionally, other monomers, and subsequentlypolymerizing the mixture. In any of the above type resins, all or aportion of the styrene may be replaced with its closely relatedhomologues such as alpha-methylstyrene, o-, m-, and p-methylstyrenes,o-, m-, and p-ethylstyrenes, 2,4-dimethylstyrene, bromostyrene,chlorostyrene, and the like. Copolymers of alkenyl aromatic, e.g.styrene, and alkenyl nitrile, e.g., acrylonitrile can also be used andcan have a weight ratio of styrene to acrylonitrile of 95:5 to 5:95respectively.

The rubber-containing blends can have the diene rubber moiety present inamounts of about 1 to 35% of grafted diene rubber particles dispersed ina matrix polymer or copolymer as a polyblend. Generally, the rubberparticles are grafted with the polymers having the same composition asthe matrix phase. The diene rubbers can be polybutadiene or copolymerrubbers having at least 50% by weight of a diene monomer, e.g.butadiene, chloroprene, isoprene or pentadiene. Comonomerscopolymerizable with the diene monomers can be those disclosed above.The copolymer rubbers may be interpolymers or block copolymers.

The masterbatch mix is a plasticizer which improves the flowcharacteristics of the foam.

The masterbatch mix comprises in its broadest aspects:

(a) a styrene resin;

(b) a rubbery block copolymer; and

(c) a solid blowing agent.

An essential element of the masterbatch mix is the styrene resin. Allcommercially available styrene polymers can be used as the styreneresin. However, it is preferable that the Vicat softening temperature ofthe chosen styrene polymer be between 45 and 82 at 50° C./hr. rise.Preferred as the styrene resin is alpha-methylstyrene. Commerciallyknown alpha-methylstyrenes include Amoco's Resin 18-240, Resin 18-210and Resin 18-290, the preferred being the Resin 18-240 which has a Vicatsoftening temperature of 60.5 at 50° C./hr. rise and 62.9 at 120° C./hr.rise.

The foregoing alpha-methylstyrenes have the following approximatephysical characteristics:

Resin 18-240 is a linear homopolymer of alpha-methylstyrene having amolecular weight of about 685, a softening point of about 99° C., and aflash point of about 210° C.; Resin 18-210 is a linear homopolymer ofalpha-methylstyrene having a molecular weight of about 790, a softeningpoint of about 118° C., and a flash point of about 224° C.; and Resin18-290 is a linear homopolymer of alpha-methylstyrene having a molecularweight of about 960, a softening point of about 141° C., and a flashpoint of about 246° C.

Another essential element of the masterbatch mix is the rubbery blockcopolymer. These are known in the art generally as having the formulae:A-B, A-B-A, A-B-A-B, and the like, including graft and radial blockcopolymers, as well as block copolymers containing other types ofblocks, "C". These rubbery block copolymers of the above formulaegenerally contain a styrenic polymer as the "A" block, and generallycontain a rubbery polymer, e.g. butadiene, ethylene/propylene,ethylene/butylene, isoprene, as the "B" block Block "C", when present,may be either a second, different styrenic polymer from the "A" block ora second, different rubbery polymer from the "B" block, as the case maybe. Preferred as the rubbery block copolymer in the masterbatch mix arethose block copolymers available from Shell Chemical Company under thedesignations "Kraton G" and "Kraton D", such as Kraton D-1101, KratonD-1102 Kraton D-1107, Kraton G-1650, Kraton G-1651, Kraton G-1652,Kraton G-1657X, Kraton G-1701X, and Kraton G-1726X. Especially preferredare Kraton G-1650 and Kraton G-1652.

The foregoing rubbery block co-polymers have the following approximateblock and physical characteristics: a styrene-butadiene-styrene blockcopolymer having a styrene/rubber ratio of about 31/69 (Kraton D-1101);a styrene-butadiene-styrene block copolymer having a styrene/rubberratio of about 28/72 (Kraton D-1102); a styrene-isoprene-styrene blockcopolymer having a styrene/rubber ratio of about 14/86 (Kraton D-1107);a styrene-ethylene/butylene-styrene block copolymer having astyrene/rubber ratio of about 29/71 (Kraton G-1650); astyrene-ethylene/butylene-styrene block copolymer having astyrene/rubber ratio of about 32/68 (Kraton G-1651); astyrene-ethylene/butylene styrene block copolymer having astyrene/rubber ratio of about 29/71 and a ring and ball softening point(ASTME 28-67, 10% by weight in Kaydol oil) of about 141° F. (KratonG-1652); a styrene-ethylene/butylene-styrene block copolymer having astyrene/rubber ratio of about 13/87 (Kraton G-1657X); astyrene-ethylene/propylene block copolymer having a styrene/rubber ratioof about 37/63 (Kraton G-1701X); and a styrene-ethylene/butylene blockcopolymer having a styrene/rubber ratio of about 30/70 (Kraton G-1726X).

The solid blowing agents which can be used in the masterbatch mix arealso known in the art and include mixtures of one or more solid organicacids, for example, oxalic acid, succinic acid, adipic acid, phthalicacid, and preferably citric acid; and an alkali metal carbonate oralkali metal bicarbonate, for example, sodium carbonate, potassiumcarbonate, and preferably sodium bicarbonate. The acid and carbonateand/or bicarbonate are generally used in alkali:acid equivalent ratiosof from about 1:3 to about 3:1, acid to carbonate (and/or bicarbonate),and are preferably used in approximate stoichiometric amounts, i.e.about 0.7 to 1.3 alkali equivalents per acid equivalent, preferablyabout 0.9 to 1.1 alkali equivalents per acid equivalent. Especiallypreferred as the solid blowing agent of the masterbatch mix arecombinations of monosodium citrate and sodium bicarbonate, preferablyencapsulated in vegetable oil (i.e. a mixture of mono-, di-, andtriglycerides), the amounts of monosodium citrate and sodium bicarbonatepresent preferably also as a stoichiometric mixture. The most preferredsolid blowing agents are the SAFOAM P and SAFOAM FP powders, availablefrom Reedy International Corporation, Keyport, N.J.

The masterbatch mix also preferably contain a lubricant/plasticizer.Suitable lubricant/plasticizers are known to those in the art andinclude paraffin oil, silicone oil, medium to long chain alkyl esters ofphthalic acid or isophthalic acid, propylene oxide and/or mineral oil.Preferred as the lubricant/plasticizer in the masterbatch mix is whitemineral oil.

Also preferably used in the masterbatch mix is a quantity of silica,which can either be incorporated into pellets of the masterbatch mix, ordusted over the surface thereof.

The masterbatch mix preferably comprises essentially about 1 to 20weight percent of stoichiometric amounts of monosodium citrate andsodium bicarbonate encapsulated in vegetable oil (preferably a mixtureof mono-, di-, and triglycerides), about 3 to 50 weight percent ofstyrene-ethylene/butylene-styrene block copolymer, about 20 to 80 weightpercent of alpha methyl styrene, about 1 to 20 weight percent of whitemineral oil and about 0.2 weight percent of silica (which acts as anucleating agent and aides in maintaining the free flow capability ofthe masterbatch mix under long term storage conditions). Among thepreferred masterbatch mixes of the present invention are those availablefrom Reedy International Corporation which are sold under the trademarksSAFOAM P-20, SAFOAM FP-20, SAFOAM FP-40, SAFOAM P-50 and SAFOAM FP-50.

SAFOAMP-20 and SAFOAMFP-20 contain about 19.8% of an equimolarcombination of monosodium citrate and sodium bicarbonate encapsulated invegetable oil (SAFOAM P and SAFOAM FP, respectively, a combination of14% mono-) 12% di-, and 72% triglycerides, 67.5% of alpha-methylstyrene(Amoco resin 18-240), about 10% of a combination ofstyrene-ethylene/propylene block copolymer (Shell Chemical Company,Kraton G-1726X) and styrene-ethylene/butylene-styrene block copolymer(Shell Chemical Company, Kraton G-1650), about 2.5% of white mineraloil, and about 0.2% of silica (predominantly present as a dusted coatingon the outside of pellets made from the remaining ingredients). SAFOAMFP-40 contains about 38.8% of an equimolar combination of monosodiumcitrate and sodium bicarbonate encapsulated in vegetable oil (SAFOAM FP,available from Reedy International Corporation), 36.6% ofalpha-methylstyrene (Amoco resin 18-240), 14.4% ofstyrene-ethylene/butylene-styrene block copolymer (Shell ChemicalCompany, Kraton G-1652), about 9.0% of white mineral oil and about 0.2%silica. SAFOAM P-50 comprises about 54.8% of an equimolar combination ofmonosodium citrate and sodium bicarbonate encapsulated in vegetable oil(SAFOAM P, available from Reedy International Corporation), about 30.5%of alpha-methylstyrene (Amoco Resin 18-240), about 12% ofstyrene-ethylene/butylene-styrene block copolymer (Shell ChemicalCompany, Kraton G-1650), about 7.5% of white mineral oil, and about 0.2%of silica.

The masterbatch mix, when used, is present in an amount of about 0.001to about 1.0% by weight, based upon the weight of the polyalkenylaromatic resin, preferably is present in an amount of about 0.01 toabout 0,035 weight percent, based upon the weight of the resin, and morepreferably is present in an amount of about 0.02 to about 0.03% byweight, based upon the weight of the resin.

The non-solid blowing agent combination of the present invention iscomprised of atmospheric gas and organic gas. The atmospheric gas andorganic gas may be added or injected into the melt either as a blend, orconcurrently, or sequentially. The non-solid blowing agent can also beadded to the melt in either gaseous or liquid forms, or combinationsthereof. The amount of non-solid blowing agent combination which can beadded in the process of the present invention ranges from about 2 toabout 20% by weight, based upon the weight of the resin. Preferably,non-solid blowing agent combination is added in an amount of about 3 to10% by weight, based upon the weight of the resin and, more preferably,from about 4 to about 7% by weight, based upon the weight of the resin.

As atmospheric gases, there can be used any of the gases normallypresent in the atmosphere, such as carbon dioxide, nitrogen, argon,helium, or neon, with carbon dioxide and nitrogen being preferred. Asthe organic gases, there can be used: the C₄ -C₆ alkanes, known to thoseskilled in the art, including butane, isobutane, pentane, neopentane,isopentane, and hexane; the chlorinated hydrocarbons (CHCs), such asmethyl chloride, methylene chloride and methyl chloroform; chlorinatedfluorocarbons (CFCs), such as trichlorofluoromethane(CFC-11),dichlorofluoromethane (CFC-12),1,1,2-trichloro-1,2,2-trifluoroethane (CFC-113),1,2,-dichloro-1,1,2,2-tetrafluoroethane (CFC-114),1-chloropentafluoroethane (CFC-115) bromochlorodifluoromethane(Halon-1211), bromotrifluoromethane (Halon-1301), anddibromotetrafluoroethane (Halon-2402) the hydrogen-containingchlorofluoro carbons (HCFCs), such as chloro difluoromethane (HCFC-22),1,1-difluoro-1,1-chloroethane (HCFC-142b); and the hydrogen-containingfluorocarbons (HFCs), such as 1,1-difluoroethane (HFC-152a),1,1,1,2-tetrafluoroethane (HFC-134a), 1,1,1,-trifluoroethane (HFC-143a)and dichlorotrifluoroethane (HFC-123). The atmospheric gas and organicgas can be present in any relative amounts, such as a ratio from 1 to 99parts by weight atmospheric gas and 99 to 1 parts by weight organic gas.Preferably, the amount of atmospheric gas is present in an amount ofgreater than 30% by weight, more preferably present in an amount ofgreater than 40% by weight, and still more preferably present in anamount of about 50% by weight or more. Most preferably, the ratio ofatmospheric gas to organic gas (based upon the weight of thecombination) is from about 35/65 to about 65/35, preferably 40/60 to60/40, and more preferably about 50/50.

The use of an extrusion process for the manufacture of alkenyl aromaticfoam is typical, but is not required. Such a process includes a primaryextruder, a blowing agent addition system, a secondary extruder, anannular die, a sheet cutter or slitter and a sheet gathering device.However, the use of this exact equipment set up is not required in theprocess of this invention.

In the preferred embodiments of the present invention, polystyrene foamis formed in a continuous process by delivering a well-mixed and uniformblend of styrenic polymer and masterbatch mix to the extruder throat.Masterbatch mix is preferably about 0.02 to 0.03% by weight of thestyrenic polymer. Once in the screw, while being rotated at a controlledRPM, the blend or feed of styrenic polymer and masterbatch mix is heatedto a temperature above the melting point of the blend, about 250° to500° F. It is then delivered with the use of relatively stable pressurein the range of about 4000-6000 psi, to the point of injection. Here, aninjection system delivers atmospheric gas, e.g. carbon dioxide in gas orliquid form, and/or nitrogen in gas form, or combinations thereof, intothe melted feed. In combination with the atmospheric gas, and in apreferred embodiment sequentially with respect to the delivery of theatmospheric gas, the injection system delivers an organic gas, e.g.isopentane or HFC 152(a), into the melted feed.

Next, the injected melted feed is passed into a second extruder. Thisextruder is designed for maximum cooling capability. It is of largercapacity than the first extruder. In this extruder, a minimum of shearis desired. Minimum shear is achieved by keeping the screw's roofdiameter constant. The injected melted feed is mixed in this secondextruder and cooled.

The feed then exits this second extruder through a die at a temperatureat or above 250° F., preferably between about 250°-290° F., and morepreferably at a temperature of between about 250° F.-280° F. and apressure of about 2,500-3,750 psi. The extruded material is stretchedout over a cooling drum and drawn to the desired thickness. Thepolystyrene foam sheet is then slit and can be wound into large rolls.

The foams produced according to the present invention are characterizedby having densities generally greater than about 2.5 pounds per cubicfoot, more preferably greater than about 3.0 pounds per cubic foot,still more preferably greater than about 3.5 pounds per cubic foot, mostpreferably greater than about 4.5-5.0 lbs. per cubic foot, andespecially preferably greater than about 6 pounds per cubic foot. Onaverage, the density of the foams according to the present inventionrange between about 4 pounds per cubic foot and about 10 pounds percubic foot, and more generally range from about 4 pounds per cubic footto 6 pounds per cubic foot.

The foams produced according to the present invention are alsocharacterized by having a substantial plurality of closed,non-interconnecting gas-containing cells. Generally, the number of suchclosed cells in the foams according to the present invention is greaterthan 50% of all of the cells present, preferably greater than 60%, morepreferably greater than 70%, still more preferably greater than 80%,especially preferably greater than 90%, and most especially preferablygreater than 95%. The gas contained in the closed cells is comprised ofa combination of atmospheric gas and organic gas and, preferably,contains combinations of atmospheric gas and organic gas, based on theweight of atmospheric gas an organic gas in the cells, having thecomponents and, in the ratios, described above for the non-solid blowingagents which are utilized according to the present invention. Forexample, a foam resulting from the process of the present invention mayhave a percentage of closed cells greater than 70%, within which theremay be a combination of gases comprised of carbon dioxide and isopentanein a weight ratio of about 40/60 based on the total weight of carbondioxide and isopentane.

The foams of the present invention are further characterized in thatthey retain the injected gas, and particularly the organic gas, to amuch greater degree than the foams of the prior art. The retainment ofthe injected gas is believed to be a function of several parameters,including the solubility of the organic gas in the foamed polymer andthe percentage of closed cells in the foam. It is also theorized thatthe use of the rubbery block copolymer in the masterbatch mix of thepreferred embodiments of the present invention aids in blocking andretaining within the foamed polymer the carbon dioxide and any watervapor which may be present in the polymer. The percentage of retainedgas in the foams according to the present invention generally coincideswith the percentage of closed cells. Thus, the percentage of gasretained in the foams according to the present invention is generallygreater than 50%, preferably greater than 60%, more preferably greaterthan 70%, still more preferably greater than 80%, especially preferablygreater than 90%, and most especially preferably greater than 95%. Thepercentage of closed cells can be measured by a Beckman Pycnometer. Dueto the increased gas retention of the foams according to the presentinvention, there is a concomitant increase in the thermoforming outputfor the produced foams. The thermoforming output (or speed at which thefoams may be thermoformed into finished product) is a function of anumber of factors as well, including cell uniformity and percentage ofclosed cells. The foams according to the present invention attaingreater than 25% improvement in thermoforming output rate as compared toprior art foams and can attain a 35%, or 50%, or greater, improvement ofthermoforming output rate.

Still further, the foams obtained through the use of the masterbatch mixaccording to the preferred processes of the present invention providesfor other significant economic improvements. Specifically, theproduction (or throughput) rate of foams according to the presentinvention when a masterbatch mix is used in the process, andparticularly when the preferred SAFOAM masterbatch mixes are utilized,is generally greater than 200 pounds of foamed product per hour,preferably is greater than 400 pounds of foamed product per hour, morepreferably greater than 500 pounds of foamed product per hour, and mostpreferably greater than 700 or 800 pounds of foamed product per hour.

In addition, the foams of the present invention, because of the improvedgas retention characteristics of the foams when the masterbatch mix isused, exhibit post-expansion characteristics which are likewise greatlyimproved. The foams according to the present invention which are made byprocesses which utilize the masterbatch mix, and preferably the SAFOAMmasterbatch mix, exhibit post-expansion properties of greater than 200%,preferably greater than 250%, more preferably greater than 300%, mostpreferably greater than 350%, especially preferably greater than 400%,and most especially preferably greater than about 450 to 500%. Thispost-expansion improvement provides for the ability to use a muchthinner starting foam product in the post-extrusion thermoformingprocess.

Another beneficial aspect of the use of the masterbatch mix according tothe present process, and especially the SAFOAM masterbatch mix, is thatit is possible to reduce by at least 5%, preferably at least 10%, andmore preferably up to about 20% or more, the amount of organic gasutilized in the process, while maintaining the same density of thefoamed product.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following examples are intended to merely illustrate the presentinvention, which is not limited thereby or thereto.

EXAMPLE 1

A virgin polystyrene resin having a weight average molecular weight ofabout 310,000 and SAFOAM P50 in an amount of 0.25 parts per 100, basedon 100 parts of resin weight were combined.

These ingredients were uniformly blended and were added to an extruder.The mix was heated to 426° F. and melted under an injection pressureranging from 4250 to 4300 psi. At this point, isopentane at 3.65% andcarbon dioxide at 2.43% (both based on 100 parts resin weight) weredelivered into the melt.

Next, the melt passed into a cooling extruder, cooled down to a die melttemperature of 274° F. and a die pressure of 2830 psi (die diameter 8")and extruded. The extruded material was stretched out over a coolingdrum with a 24.9" diameter and drawn to the physical parameters listedbelow.

Percent of closed cells=98.5%

Post-expansion (1 hr. after extrusion)=285%

The polystyrene foam was extruded with the following process andphysical parameters:

    ______________________________________                                        Total Output       825 lbs./hr.                                               Sheet Cross Sectional                                                                            .105"                                                      Thickness                                                                     Sheet Density      3.8 lbs./ft..sup.3                                         Blowing Agent                                                                 Isopentane                                                                                        ##STR1##                                                  CO.sub.2                                                                                          ##STR2##                                                  This represents a ratio of 60% isopentane to 40% CO.sub.2.                    Masterbatch P50                                                                                   ##STR3##                                                  ______________________________________                                    

After aging the foamed sheet for three days, it was passed through athermoformer at 32 cycles per minute; forming meat trays with a bottomcross sectional thickness ranging from 0.165" to 0.170". The percentageof isopentane retained in the formed trays was 3.32 lbs., based on 100lbs. of resin, giving an isopentane retention of 91.1%.

EXAMPLE 2

A virgin polystyrene resin having a weight average molecular weight ofabout 310,000 of SAFOAM P50 in an amount of 0.25 parts per 100, based on100 parts of resin weight were combined.

These ingredients were uniformly blended and were added to an extruder.The mix was heated to 435° F. and melted under an injection pressureranging from 4300 to 4370 psi. At this point, isopentane at 3.01% andcarbon dioxide at 1.9% (both based on 100 parts resin weight) weredelivered into the melt.

Next, the melt was passed into a cooling extruder, cooled down to a diemelt temperature of 268° F. and a die pressure of 2490 psi (die diameter8").

The extruded material was stretched out over a cooling drum with a 24.9"diameter and drawn to the physical parameters listed below.

Percent of closed cells=96.7%

Post-expansion (1.5 hr. after extrusion)=295%

Same extruded polystyrene foam with the following components:

    ______________________________________                                        Total Output       825 lbs./hr.                                               Sheet Cross Sectional                                                                            .095"                                                      Thickness                                                                     Sheet Density      4.8 lbs./ft..sup.3                                         Blowing Agent                                                                 Isopentane                                                                                        ##STR4##                                                  CO.sub.2 Gas                                                                                      ##STR5##                                                  This represents a ratio of 60% isopentane to 40% CO.sub.2.                    Masterbatch P50                                                                                   ##STR6##                                                  ______________________________________                                    

After aging the foamed sheet for three days, it was passed through athermoformer at 30 cycles per minute, forming large meat trays with abottom cross sectional thickness ranging from 0.170% to 0.173%. Thepercentage of isopentane retained in the formed trays was 2.79 lbs.,based on 100 lbs. of resin, an isopentane retention of 93.0%.

EXAMPLE 3

A virgin polystyrene resin having a weight average molecular weight ofabout 310,000 and recycled polystyrene having a weight average molecularweight of about 290,999 at amounts ranging about 9:1 (virgin:recycled)based on weight and SAFOAM P-50 in an amount of 0.23 parts per hundred,based on 100 parts of resin weight, were combined.

These ingredients were uniformly blended and added to an extruder. Themix was heated to 435° F. and melted under an injection pressure rangingfrom 4550 to 4600 psi. At this point, HFC-152a at 2.6% and carbondioxide at 2.4% (both based on 100 parts by weight virgin plus recycled)was delivered into the melt.

Next, the melt was passed into a cooling extruder, cooled down to a diemelt temperature of 284° F. and a die pressure of 3250 psi (die diameter8") and extruded.

The extruded material was stretched out over a cooling drum with a 26.1"diameter and drawn to the physical parameters listed below.

Percent of closed cells=71.2%

Post-expansion (1.0 hr. after extrusion)=250%

After aging the foamed sheet for two days, it was passed through athermoformer at 27 cycles per minute, forming large hinged foodcontainers with a bottom cross sectional thickness ranging from 0.105%to 0.110%.

Same extruded polystyrene foam with the following components:

    ______________________________________                                        Total Output       725 lbs./hr.                                               Sheet Cross Sectional                                                                            .08"                                                       Thickness                                                                     Sheet Density      5.2 lbs./ft..sup.3                                         Blowing Agent                                                                 HFCs 152A                                                                                         ##STR7##                                                  CO.sub.2 Gas                                                                                      ##STR8##                                                  This represents a ratio of 52% HFC 152A to 48% CO.sub.2.                      Masterbatch P50                                                                                   ##STR9##                                                  ______________________________________                                    

The present invention has been described with respect to the preferredembodiments. It is to be understood, however, that modifications andvariations may be resorted to, without departing from the spirit andscope of the invention, as those skilled in the art would readilyunderstand. These modifications and variations are considered to bewithin the scope of the appended claims.

All of the above-mentioned patents and publications are incorporatedherein by reference.

What is claimed is:
 1. A process for the production of alkenyl aromaticfoams, said process comprising:(a) heating an alkenyl aromatic resin toa temperature above its melting point to form a melted alkenyl aromaticresin; (b) adding to the melted alkenyl aromatic resin a masterbatch mixcomprised of:(i) a styrene resin; (ii) a rubbery block copolymer; and(iii)a solid blowing agent to form an alkenyl aromatic/masterbatch mixblend; (c) heating the alkenyl aromatic/masterbatch mix blend to atemperature sufficient to form a melted blend; (d) injecting into themelted blend a non-solid blowing agent comprised of a combination ofatmospheric gas and organic gas to form an injected melted blend; (e)mixing the injected melted blend to form a mixed injected melted blend;and (f) cooling and extruding the mixed injected melted blend as analkenyl aromatic foam.
 2. A process according to claim 1, wherein instep (a) the alkenyl aromatic resin is comprised of styrene.
 3. Aprocess according to claim 1, wherein in step (a) the alkenyl aromaticresin is comprised of a copolymer or interpolymer of styrene containinggreater than 75 weight percent styrene.
 4. A process according to claim1, wherein in step (a) the alkenyl aromatic resin is comprised of ablend of styrene and a rubbery polymer.
 5. A process according to claim1, wherein in step (a) the styrene resin has a Vicat softeningtemperature of between 45 and 82 at 50° C./hr. rise.
 6. A processaccording to claim 1, wherein in step (b) the styrene resin is comprisedof alpha-methylstyrene.
 7. A process according to claim 6, wherein thealpha-methylstyrene is selected from the group consisting of: a linearhomopolymer of alpha-methylstyrene having a molecular weight of about685, a softening point of about 99° C., and a flash point of about 210°C., a linear homopolymer of alpha-methylstyrene having a molecularweight of about 790, a softening point of about 118° C., and a flashpoint of about 224° C. and a linear homopolymer of alpha-methylstyrenehaving a molecular weight of about 960, a softening point of about 141°C., and a flash point of about 246° C.
 8. A process according to claim1, wherein in step (b) the rubbery block copolymer is selected from thegroup consisting of A-B, A-B-A, A-B-A-B, graft and radial blockcopolymers.
 9. A process according to claim 1, wherein in step (b) therubbery block copolymer is selected from the group consisting of: astyrene-butadiene-styrene block copolymer having a styrene/rubber ratioof about 31/69 (Kraton D-1101); a styrene-butadiene-styrene blockcopolymer having a styrene/rubber ratio of about 28/72(Kraton D-1102); astyrene-isoprene-styrene block copolymer having a styrene/rubber ratioof about 14/86 (Kraton D-1107); a styrene-ethylene/butylene-styreneblock copolymer having a styrene/rubber ratio of about 29/71. (KratonG-1650); a styrene-ethylene/butylene-styrene block copolymer having astyrene/rubber ratio of about 32/68 (Kraton G-1651); astyrene-ethylene/butylene styrene block copolymer having astyrene/rubber ratio of about 29/71 and a ring and ball softening point(ASTME 28-67, 10% by weight in Kaydol oil) of about 141° F. (KratonG-1652); a styrene-ethylene/butylene-styrene block copolymer having astyrene/rubber ratio of about 13/87 (Kraton G-1657X); astyrene-ethylene/propylene block copolymer having a styrene/rubber ratioof about 37/63 (Kraton G-1701X); and a styrene-ethylene/butylene blockcopolymer having a styrene/rubber ratio of about 30/70 (Kraton G-1726X).10. A process according to claim 9, wherein the rubbery block copolymeris selected from the group consisting of: astyrene-ethylene/butylene-styrene block copolymer having astyrene/rubber ratio of about 29/71 (Kraton G-1650) and astyrene-ethylene/butylene styrene block copolymer having astyrene/rubber ratio of about 29/71 and a ring and ball softening point(ASTME 28-67, 10% by weight in Kaydol oil) of about 141° F. (KratonG-1652).
 11. A process according to claim 1, wherein in step (b) thesolid blowing agent comprises a mixture of (1) one or more solid organicacids and (2) an alkaline metal carbonate or alkaline metal bicarbonate.12. A process according to claim 11, wherein the one or more solidorganic acids is selected from the group consisting of oxalic acid,succinic acid, adipic acid, phthalic acid and citric acid.
 13. A processaccording to claim 11, wherein the alkaline metal carbonate or alkalinemetal bicarbonate is selected from the group consisting of sodiumcarbonate, potassium carbonate and sodium bicarbonate.
 14. A processaccording to claim 11, wherein the alkali:acid equivalent ratios arefrom about 1:3 to about 3:1.
 15. A process according to claim 14,wherein the alkali:acid equivalent ratio is from about 0.7:1 to 1.3:1.16. A process according to claim 11, wherein the solid blowing agent iscomprised of a combination of monosodium citrate and sodium bicarbonate.17. A process according to claim 16, wherein the combination ofmonosodium citrate and sodium bicarbonate are encapsulated in vegetableoil and the alkali:acid equivalent ratio is from about 0.9:1 to about1.1:1.
 18. A process for the production of alkenyl aromatic foams, saidprocess comprising:(a) heating an alkenyl aromatic resin to atemperature above its melting point to form a melted alkenyl aromaticresin; (b) adding to the melted alkenyl aromatic resin a masterbatch mixcomprised of:(i) from about 1 to about 55 weight percent ofstoichiometric amounts of monosodium citrate and sodium bicarbonateencapsulated in vegetable oil; (ii) about 3 to 50 weight percent ofstyrene-ethylene/butylene-styrene block copolymer; (iii) about 20 to 80weight percent of alpha-methylstyrene; (iv) about 1 to 20 weight percentof white mineral oil; and (v) about 0.2 weight percent of silica; (c)heating the alkenyl aromatic/masterbatch mix blend to a temperaturesufficient to form a melted blend; (d) injecting into the melted blend anon-solid blowing agent comprised of a combination of atmospheric gasand organic gas to form an injected melted blend; (e) mixing theinjected melted blend to form a mixed injected melted blend; and (f)cooling and extruding the mixed injected melted blend as an alkenylaromatic foam.
 19. A process according to claim 18, wherein in step (a),the alkenyl aromatic resin is comprised of styrene.
 20. A processaccording to claim 18, wherein in step (a) the alkenyl aromatic resin iscomprised of a copolymer or interpolymer of styrene containing greaterthan 75 weight percent styrene.
 21. A process according to claim 18,wherein in step (a) the alkenyl aromatic resin is comprised of a blendof styrene and a rubbery polymer.
 22. A process according to claim 18,wherein in step (b) the masterbatch mix is comprised of:(i) about 19.8%to about 54.8% of an equimolar combination of monosodium citrate andsodium bicarbonate encapsulated in vegetable oil; (ii) about 10% toabout 14.4% of styrene-ethylene/butylene-styrene block copolymer, alone,or in combination with styrene-ethylene/propylene block copolymer; (iv)from about 30.5% to about 67.5% of alpha-methylstyrene; (v) about 2.5%to about 9% white mineral oil; and (vi) about 0.2% of silica.
 23. Aprocess according to claim 18, wherein the masterbatch mix is present inan amount of about 0.001 to about 1% by weight, based upon the weight ofthe alkenyl aromatic resin.
 24. A process according to claim 22, whereinthe masterbatch mix is present in an amount of about 0.01 to about 0.035weight percent, based upon the weight of the alkenyl aromatic resin. 25.A process according to claim 18, wherein in step (d) the amount ofnon-solid blowing agent combination added is from about 2 to about 20%by weight, based upon the weight of the alkenyl aromatic resin.
 26. Aprocess according to claim 25, wherein the amount of non-solid blowingagent combination added is from about 3 to about 10% by weight, basedupon the weight of the alkenyl aromatic resin.
 27. A process accordingto claim 18, wherein in step (d) the atmospheric gas is selected fromcarbon dioxide, nitrogen and mixtures thereof.
 28. A process accordingto claim 18, wherein in step (d) the organic gas is selected from thegroup consisting of C₄ -C₆ alkanes, chlorinated hydrocarbons,chlorinated fluorocarbons, hydrogen-containing chlorofluoro carbons,hydrogen-containing fluorocarbons and mixtures thereof.
 29. A processaccording to claim 28, wherein the organic gas is selected from thegroup consisting of butane, isobutane, pentane, neopentane, isopentane,hexane, methyl chloride, methylene chloroform, trichlorofluoromethane(CFC-11), dichlorofluoromethane (CFC-12),1,1,1,2-trichloro-1,2,2-trifluoroethane (CFC-113),1,1,2,-dichloro-1,1,2,2,-tetrafluoro ethane (CFC-114), 1-chloropentafluoro ethane (CFC-115), bromochlorodifluoromethane (Halon-1211),bromotrifluoromethane (Halon-1301), dibromo tetrafluoro ethane(Halon-2402), chlorodifluoromethane, 1,1-difluoro-1, 1-chloroethane,1,2-difluoroethane, 1,1,1-trifluoroethane and dichlorotrifluoroethane(HFC-123).
 30. A process according to claim 29, wherein the atmosphericand organic gases are present in a ratio of from 1:99 to 99:1.
 31. Aprocess according to claim 30, wherein the atmospheric gas is present inan amount of from greater than 30% by weight, based upon the combinationof atmospheric and organic gases.
 32. A process according to claim 31,wherein the ratio of atmospheric gas to organic gas is from about 35:65to about 65:35, based upon the weight of the combination of atmosphericgas and organic gas.
 33. A process for the production of polystyrenefoams, said process comprising:(a) mixing a polystyrene resin with amasterbatch mix comprised of:(i) about 54.8% of a combination ofmonosodium citrate and sodium bicarbonate encapsulated in vegetable oil;(ii) about 12% of styrene-ethylene/butylene-styrene block copolymer;(iii) about 30.5% of alpha-methylstyrene; (iv) about 7.5% white mineraloil; and (v) about 0.2% silica; (b) heating the polystyrene/masterbatchmix blend to a temperature sufficient to form a melted blend; (c)injecting into the melted blend a non-solid blowing agent comprised of acombination of atmospheric gas and organic gas to form an injectedmelted blend, wherein the atmospheric gas is comprised of carbon dioxideand is present in an amount of from about 1.9 to about 2.4% by weight,based upon 100% by weight of polystyrene resin, and wherein the organicgas is selected from the group consisting of isopentane and1,1-difluoroethane (HFC-152A) in an amount of from about 2.6% to 3.65%,based on 100 parts by weight of polystyrene resin to form an injectedmelted blend, based upon 100 parts by weight of polystyrene resin toform an injected melted blend; (d) mixing the injected melted blend toform a mixed injected melted blend; and (e) cooling the mixed injectedmelted blend to a temperature of between 268° F. and 284° F. andextruding the mixed injected melted blend as a polystyrene foam.
 34. Aprocess for the production of polystyrene foams, said processcomprising:(a) mixing a polystyrene resin with a masterbatch mixcomprised of:(i) about 19.8% of a combination of monosodium citrate andsodium bicarbonate encapsulated in vegetable oil; (ii) about 10% of acombination of styrene-ethylene/propylene block copolymer andstyrene-ethylene/butylene-styrene block copolymer; (iii) about 67.5% ofalpha-methylstyrene; (iv) about 2.5% of white mineral oil; and (v) about0.2% silica; (b) heating the polystyrene/masterbatch mix blend to atemperature sufficient to form a melted blend; (c) injecting into themelted blend a non-solid blowing agent comprised of a combination ofatmospheric gas and organic gas to form an injected melted blend,wherein the atmospheric gas is comprised of carbon dioxide and ispresent in an amount of from about 1.9 to about 2.4% by weight, basedupon 100% by weight of polystyrene resin, and wherein the organic gas isselected from the group consisting of isopentane and 1,1-difluoroethanein an amount of from about 2.6% to 3.65%, based on 100% by weight ofpolystyrene resin, to form an injected melted blend; (d) mixing theinjected melted blend to form a mixed injected melted blend; (e) coolingthe mixed injected melted blend to a temperature between 268° F. and284° F. and extruding the mixed injected melted blend as a polystyrenefoam.
 35. A process for the production of polystyrene foams, saidprocess comprising:(a) mixing a polystyrene resin with a masterbatch mixcomprised of:(i) about 38.8% of a combination of monosodium citrate andsodium bicarbonate encapsulated in vegetable oil; (ii) about 14.4% ofstyrene-ethylene/butylene-styrene block copolymer; (iii) about 36.6% ofalpha-methylstyrene; (iv) about 9% white mineral oil; and (v) about 0.2%silica (b) heating the polystyrene/masterbatch mix blend to atemperature sufficient to form a melted blend; (c) injecting into themelted blend a non-solid blowing agent comprised of a combination ofatmospheric gas and organic gas to form an injected melted blend,wherein the atmospheric gas is comprised of carbon dioxide and ispresent in an amount of from about 1.9 to about 2.4% by weight, basedupon 100% by weight of polystyrene resin, and wherein the organic gas isselected from the group consisting of isopentane and 1,1-difluoroethanein an amount of from about 2.6% to 3.65%, based on 100 parts by weightof polystyrene resin to form an injected melted blend; (d) mixing theinjected melted blend to form a mixed injected melted blend; (e) coolingthe mixed injected melted blend to a temperature between 268° F. and284° F. and extruding the mixed injected melted blend as a polystyrenefoam.